The invention relates generally to methods for chemical vapor deposition of a metal, particularly copper, onto a substrate.
In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are deposited upon the surface of a semiconductor substrate or wafer. The films provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via in a semiconductor substrate. The film, passing through the insulative layers of the substrate, provides plugs of conductive material for the purpose of making interconnections across the insulating layers.
One well known process for depositing a thin metal film is by chemical vapor deposition (CVD). In CVD, reactant or deposition gas precursors are pumped into proximity to a substrate inside a reaction chamber. The precursors are activated using either thermal energy or electrical energy, and subsequently undergo chemical reactions at the surface of the substrate. This results in one or more reaction by-products, which are deposited on the exposed substrate or wafer surface to form a film.
Copper CVD reactions are limited mainly by the low partial pressure of potential copper precursors. In fact, most of the potential precursors have such a low partial pressure that use of a gas mass flow controller (MFC) and vapor draw from a liquid precursor in a vessel is difficult. Instead, direct liquid injection (DLI) of the liquid copper precursor must be used.
DLI of a copper precursor, however, is undesirable for several reasons. One reason is that it requires use of two devices: a first device to form a mist or aerosol of the precursor liquid, and a second device to transform the mist into a precursor vapor. Additionally, during this process the stability of the precursor source is generally a problem; the precursor tends to decompose or change composition along the path of its liquid injection and vaporization. Another reason is that control of the deposition rate is difficult in DLI of the precursor liquid. Also, the DLI device may clog if precursor deposition occurs in the device itself. Moreover, a device such as a showerhead, which is supposed to aid in uniform distribution of precursor at the surface of the wafer or substrate, might become hot enough to participate in decomposition of the copper precursor, and the precursor may deposit on the showerhead. Such an occurrence would make control of precursor delivery to the wafer even more difficult.
What is needed is a way to generate a copper or other metal precursor vapor to avoid problems that arise with injecting a precursor in a liquid state.
The present invention is directed to a method of generating a metal precursor, such as copper precursor, used for CVD within the deposition reactor. This method generates the vapor precursor from simple elements in-situ, and thus reduces or eliminates the need to obtain a sufficient partial pressure to deliver the copper or other metal precursor. The present invention is also directed to a method of delivering the vapor precursor to the wafer to deposit a thin film. To this end, and in accordance with the principles of the present invention, a solid source of a metal in an oxidation state of 1 or greater is heated in the chamber above the substrate to a temperature at which the source will react with a gas injected into the chamber to form a vapor metal precursor. This precursor is directed toward the substrate, which is heated so as to decompose the precursor onto the substrate surface.